The present invention relates to fluid pumps and to specialized pumping assemblies such as blood pumps, left ventricular assist devices (LVADs) and artificial hearts. It particularly relates to magnetically operated rotating pumps--that is, pumps in which rotational motive power is applied to pump the fluid by an arrangement of driven magnets or electromagnets--and to rotary pumps wherein magnets serve to support and align the rotor, or function as bearings. The invention also relates to drive assemblies and control systems for rotary magnetic blood pumps.
A number of reasonably effective blood pumps are currently available in the market place for providing pumping circulation during relatively short periods, e.g., intervals of a few hours or a day, to supplement or replace normal cardiac circulatory function.
One of these pumps, known as the St. Jude pump after its developing institution, has a broad, relatively flat impeller situated within a housing that has inlet and outlet tube connector ports. The impeller has a generally disc-shaped lower body portion with vanes on its upper surface and an inlet at the center, so that blood entering at the inlet along a central rotation axis is urged radially outward by the vanes to exit at higher pressure along an outflow path at the disc periphery. A shaft extending through the bottom of the disc on the opposite side from the vaned top surface centers the assembly, with the rotation shaft and bearings being located out of the blood flow path and shielded therefrom by seals. Multiple circumferentially-spaced ferromagnetic plates are embedded in the disc body portion, and the pump assembly is driven by a separate driver unit that fastens to the housing and rotates a similarly-poled magnetic disc positioned directly below and closely parallel to the impeller so that the driver disc magnetically engages the plates on the rotor. Some construction details of this pump are further shown in U.S. Pat. No. 5,017,103.
Another currently available pump, sold by Bio-Medicus, Inc. of Minnetonka, Minn. has a rotor assembly in a housing wherein the impeller has a built-up disc body, which, rather than vanes, has several successive sheet-like curved upper surfaces that are arranged at different closely spaced heights along the vertical rotation axis, and to which the blood is delivered at the center of each surface from a central inlet. Each surface is smooth and continuous, without vanes, and the surfaces each engage blood by surface friction to carry it around and drive it outwardly, thus creating gentle pumping action which is less traumatic to blood cells; the multiple top surfaces collectively provide a large active pumping surface area. The bottom of this impeller disc is flat, and has one multi-poled magnet or a number of separate magnets embedded therein to provide six magnetic pole regions spaced at equal angular sectors. As in the St. Jude pump, the pump is driven by a drive unit that mounts with a bayonet fitting parallel to the underside of the disc to engage the impeller with a similarly-sized driver having corresponding poles, so that the magnetic coupling between the impeller and the driver causes the impeller to turn at the speed of the driver. Various aspects of this pump are described in U.S. Pat. Nos. 3,647,324; 3,864,055; 3,957,389; 3,970,408; 4,037,984; and Re 28,742.
Each of these constructions has a shaft and bearing structure which, of necessity, involves seals and generates heat that may potentially lead to blood cell injury or flow disturbances. Furthermore, each involves a certain amount of dead space which may lead to regions of flow stagnation that could engender sepsis or thrombic accumulations. Mechanical bearings may also shed lubricant or foreign particulate matter into the blood. Thus, while the permanent magnet construction of the rotor, and the rigid axial suspension with a separate drive advantageously allow the pump itself to be entirely free of internal coils and electrical feed-throughs, the rotor design and suspension retain certain conventional mechanical features which may pose risks when used as a blood pump.
It is possible to design a motor, (generator, or turbine such that the rotor is entirely magnetically suspended, as suggested for example, in U.S. Pat. No. 5,208,522. In such constructions, several different sets of magnets are arranged to provide forces to maintain a desired axial alignment, and forces for maintaining a desired radial centricity. In practice, when a fixed mechanical shaft bearing is absent, a magnetic suspension may require control of five degrees of freedom, since two tilt components must be addressed in addition to the three translational coordinates. Such technology would allow one to implement a blood pump as an essentially free-floating, magnetically suspended impeller body in a flow path. Nonetheless, the net amount of force which can be generated by a magnetic bearing is highly dependent on the magnets employed and the gaps over which they are required to act, and to apply this architecture to a blood pump would further need to address the mechanical forces caused by blood flow, as well as constraints on flow that are peculiar to blood pumping. Any active control further requires both a suitable set of position or force sensors, and an effectively implemented control regimen. These considerations all affect the weight, rotational inertia, coil size, drive current requirements and potential physical geometry or shape of the assembly, and implicate such characteristics as cost, size, energy efficiency, reliability, heat generation, thrombogenicity and the like. For these reasons, it is not immediately clear whether such a motor could be implemented, or whether a blood pump designed along these principles would have, or could have, desirable or improved operating characteristics.
One approach to building a magnetically suspended pump is shown in U.S. Pat. No. 4,944,748 and a number of later continuation patents derived therefrom. As set forth in those patents, coils in the housing of a pump body may respond to rotor position measurements derived from an LED/photodetector sensing arrangement, and from a sensed pump pressure, to determine an appropriate level of axial force to be applied to the pump rotor and then produce corresponding corrections in the power provided to magnets of the unit. Applicant is not aware whether the constructions shown in these latter patents have been implemented or tested.
In general, a great number of other considerations affecting blood flow and biological compatibility must be addressed in the construction of any particular blood pump, and the choice of providing magnetic drive and bearings may arise at a late stage in the pump design, after one or more mechanically-suspended prototypes have been tested and the overall size, shape, speed and other characteristics of a pump mechanism have been determined. This piecemeal or iterative approach, while perhaps necessary in the complex and highly risky field of designing machines for in vivo blood handling, may result in designs which are suboptimal in one or more respects. In particular, it may result in a heavy or cumbersome construction, or one which dissipates excessive heat or has an unduly complex control system. Accordingly, there is a need for a simple and effective magnetically suspended blood pump, and for a dependable pump driver.